Fuel cells directly convert chemical energy into electric energy by supplying a fuel and an oxidant to two electrically connected electrodes and electrochemically causing oxidation of the fuel. Therefore, the fuel cells, unlike heat engines of fire power plants and the like, are free from the restrictions of the Carnot cycle, and exhibits high energy conversion efficiency. Ordinarily, the fuel cells are constructed by stacking a plurality of unit cells that have, as a basic structure, a membrane-electrode assembly formed by sandwiching an electrolyte membrane between a pair of electrodes. In particular, a solid polymer electrolyte fuel cell incorporating a solid polymer electrolyte membrane as an electrolyte membrane is drawing attention particularly as a power source for portable or mobile appliances due to its advantages of being easy to miniaturize, being operable at low temperature, etc.
On the anode (fuel electrode) in the solid polymer electrolyte fuel cell, the reaction of the formula (1) progresses.H2→2H++2e−  (1)
The electrons generated in the reaction of the formula (1) move through an external circuit, and reach the cathode (oxidant electrode) after doing work in a load provided outside. The protons generated in the reaction of the formula (1) move in a hydrated form within a solid polymer electrolyte membrane from the anode side to the cathode side due to electroosmosis. Then, on the cathode, the reaction of the formula (2) progresses.4H++O2+4e−→2H2O  (2)
Generally, a solid polymer electrolyte fuel cell has a structure in which electrodes each having a catalyst layer that contains carbon-based particles of carbon black or the like loaded with a catalyst, such as platinum or the like, and that also contains an electrolyte resin, and a gas diffusion layer made of an electrically conductive porous body, such as a carbon cloth or the like, are disposed on both sides of an electrolyte membrane (see FIG. 3). When the reaction of the formula (1) progresses on the catalyst in the anode of the solid polymer electrolyte fuel cell, the protons generated from hydrogen move through the electrolyte resin to the electrolyte membrane, and then reach the cathode. Then, the protons that have reached the cathode move through the electrolyte resin to the catalyst in the cathode. On the other hand, the electrons generated from hydrogen at the anode move through the carbon-based particles to the current collector, and pass through the external circuit and reach the cathode, and then move along carbon-based particles to the catalyst in the cathode.
Each of these electrochemical reactions in the anode and the cathode actively progresses at an interface where there coexist three phases: the catalyst that accelerates the reaction, the carbon-based particles that conduct electrons, and the electrolyte resin that conducts protons. The reactions in the anode-side and cathode-side catalyst layers become more active and therefore the power generation performance of the cell becomes higher the greater the amount of the catalyst that is supported on carbon particles. However, since the catalyst used in the fuel cell is a noble metal such as platinum or the like, there is a problem of the production cost of the fuel cell increasing if the amount of the catalyst supported is increased.
In the reaction electrodes in which the catalyst is supported on carbon particles, loss in the electron conduction occurs between carbon particles, and between a carbon particle and a separator that is a current collector. This electron loss is considered a cause of a performance ceiling in the electric power generation. Therefore, three of the present inventors, and others have developed a fuel cell as shown in FIG. 4 whose electrode has a structure in which carbon nanotubes are oriented substantially vertically to an electrolyte membrane, and surfaces of the carbon nanotubes are loaded with a catalyst, and are coated with an electrolyte resin (Japanese Patent Application Publication No. JP-A-2005-4967). The electrode having the structure in which carbon nanotubes are vertically oriented is excellent in electron conductivity, and restrains the electron loss in comparison with the case where the catalyst is supported on carbon particles. Therefore, the power generation efficiency per catalyst weight improves.
However, it has been reported that the electrolyte resin adjacent to the catalyst needs to have a certain membrane thickness in order that the protons generated on the catalyst move (e.g., see “Membrane Thickness Dependency of the Cast Nafion Thin Membrane Ion Conductivity” Shiroma et al, the Cell Symposium 3C10 (2003)). Thus, if the thickness of the electrolyte resin applied to a carbon nanotube surface is thin, there is a greater resistance to movement of the protons which are generated on the catalyst supported relatively close to an end portion of the carbon nanotube opposite from the side that is in contact with the electrolyte membrane, and which move through the electrolyte membrane applied thinly on the carbon nanotube surface to reach the electrolyte membrane. Likewise, if the thickness of the electrolyte resin applied to a carbon nanotube surface is thin, there is a greater resistance to the protons moving from the electrolyte membrane through the electrolyte resin applied thinly on the carbon nanotube surface to reach the catalyst supported relatively close to an end portion of the carbon nanotube opposite from the side thereof that is in contact with the electrolyte membrane.
If the conductivity of protons from the catalyst present at a location remote from the electrolyte membrane or to the catalyst is low, the catalyst supported on the carbon nanotubes which effectively contributes to the electrode reactions is limited to the catalyst supported on a side portion of each carbon nanotube that is in contact with the electrolyte membrane, and thus the catalyst utilization efficiency declines. As a method for restraining the decline of the catalyst utilization rate, Japanese Patent Application Publication No. JP-A-2005-4967 describes an arrangement in which the catalyst is supported on a side of a carbon nanotube that is in contact with the electrolyte membrane. However, in this method, while the catalyst utilization rate improves, the power generation efficiency per unit volute of the fuel cell is constrained.
Furthermore, an attempt to improve the proton conductivity by filling the spaces of the vertically oriented carbon nanotubes with an electrolyte resin faces a problem of the gas diffusivity declining so that the reactant gas fails to reach the catalyst.